369 

y l 



UNIVERSITY OF CALIFORNIA PUBLICATIONS 

IN 

AGRICULTURAL SCIENCES 

Vol. 3, No. 3,'pp. 37-54, plate 12 September 29, 1917 



SOME ABNORMAL WATER RELATIONS IN 
CITRUS TREES OF THE ARID SOUTH- 
WEST AND THEIR POSSIBLE 
SIGNIFICANCE 



BY 

ROBERT W. HODGSON 



UNIVERSITY OF CALIFORNIA PRESS 
BERKELEY 



Mono. 



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west and their Possible Significance, by Bobert W. Hodgson. Pp. 
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UNIVERSITY OF CALIFORNIA PUBLICATIONS 

IN 

AGRICULTURAL SCIENCES 

Vol. 3, No. 3, pp. 37-54, plate 12 September 29, 1917 



SOME ABNORMAL WATER RELATIONS IN 
CITRUS TREES OF THE ARID SOUTH- 
WEST AND THEIR POSSIBLE 
SIGNIFICANCE 

BY 

EOBEET W. HODGSON 



INTRODUCTION 

The progress of the development of the citrus industry, in general, 
and that of California in particular, has frequently been retarded or 
temporarily stopped by serious obstacles in the form of insect pests 
or plant diseases. Some of the most baffling of these troubles fall 
naturally into a group which for want of a better name has come to 
be known as that of "physiological diseases," which are thought to be 
caused by various obscure derangements of nutrition or other vital 
functions. This group includes mottled-leaf, die-back, chlorosis, June 
drop, puffing of the fruit, and others of less importance. Knowledge 
of the true nature of this class of diseases is extremely meager in spite 
of the fact that they have received much earnest attention from scien- 
tific investigators ; and little can be accomplished in the way of devis- 
ing control measures until much more is known in regard to them. 
Nor can w r e hope to progress far beyond the realm of speculation with- 
out greatly augmenting our knowledge of the physiology and anatomy 
of the normal citrus tree when grown under any one of a series of 
very widely varying environmental complexes which obtain in differ- 
ent parts of the arid southwest. 

It is, therefore, proposed to attempt by means of a series of sys- 
tematic experimental studies to obtain some definite and accurate 
information on the physiology of the genus Citrus. It is hoped that 
the results may serve as a basis for the elucidation of some, at least, 



38 University of California Publications in Agricultural Sciences [Vol. 3 

of the important problems referred to above. The studies in ques- 
tion will attempt to shed light on transpiration problems, nutrition 
problems, and others equally important. The paper which is sub- 
mitted herewith forms an introductory contribution to the subject 
under investigation. 

The writer is not unaware of the essential similarity between the 
physiological problems presented by citrus and other fruit trees. He 
has chosen, however, to study the physiology of the citrus tree as a 
separate entity because of the reasons given above, and the further 
one that the peculiar climatic conditions under which this tree is 
frequently placed in the arid southwest, demand a special treatment. 
Doubtless much may be gained from these studies which will apply 
to physiological problems connected with other trees. 

The data here presented were obtained during an investigation of 
one of the so-called physiological diseases above mentioned, namely, 
the June drop. 1 Ever since the Washington Navel orange has been 
grown in the dry interior valleys of Arizona and California, this 
variety has been subject to excessive dropping of the young fruits. 
This has came to be known popularly as the June drop although the 
fall of the fruits is by no means confined to June but may occur at 
any time from petal fall, in April, until the fruit reaches several 
inches in diameter in August. The prevalence and amount of this 
dropping seems to be influenced to a marked degree by certain 
environmental factors to which the trees are subject. The regular 
annual shedding of the young fruits is most serious in regions where 
the annual precipitation is lowest, the mean summer temperature 
highest, atmospheric humidity lowest, solar radiation most intense, 
and air movement greatest during the growing season. That the 
excessive drop of young fruit is in some way intimately connected 
with extreme climatic conditions is indicated by the fact that in some 
parts of southern California, where the drop is ordinarily not excess- 
ive, the hot wave of June 15-17, 1917, during which a temperature 
of 118° F was experienced in the Riverside and Redlands districts, 
was immediately followed by a drop so severe that practically the 
entire young crop of navel oranges was lost. 

The experimental work from which the data were obtained was 
carried on at Edison, Kern County, California. Edison comprises 



i This investigation, which is now in progress, was carried on in collabora- 
tion with Professor J. Eliot Coit who planned the first series of experiments 
and began the work in February, 1916. A joint-authorship paper correlating 
this and other aspects of the June drop phenomenon is in course of prepara- 
tion. 



1917] Hodgson: Abnormal Water Relations in Citrus Trees 39 

a small colony of about seven hundred acres of orange orchard 
located eight miles southeast of Bakersfield and surrounded on two 
sides by typical desert of the southern San Joaquin valley, with its 
characteristic semixerophytic flora. Extreme climatic conditions, as 
above mentioned, are operative there but the Washington Navel 
orange matures early and is of excellent quality, although crops are 
small because the drop referred to is excessive. 



Water Relations and Abscission 

It has long been recognized that abnormalities or irregularities in 
the water relations of plants are often associated with the abscission 
of various plant parts. Balls 2 was able to cause complete shedding 
of leaves, flower buds, and bolls of the cotton plant Gossypium her- 
bact um within four days by pruning the roots and so limiting the 
ability of the plant to take up water. Lloyd :l in his investigation of 
the cause of abscission in the same plant came to the conclusion that 
the causative factor lay in a steady decrease in the moisture content 
of the soil in contact witli the roots of the plant. This reduction 
causes a severe tax on the power of the plant to maintain normal 
water relations and results in fluctuations in the water content of 
the aerial parts which, in turn, leads to abscission. 

Although the work of Lloyd was performed in the humid southern 
states, he makes the statement that "there seldom occurs a day on 
which there is no minus water fluctuation in the plant." He based 
this conclusion not only on data derived from shedding records but 
also on a study of transpiration rates, and water deficit in the leaves. 
In connection with his observations on the effect of temperature in 
causing acceleration of abscission, he came to the conclusion that "the 
water deficit is the cause of the rise of temperature in the tissues and 
that this constitutes the stimulus which directly leads to abscission." 

Other evidence of the occurrence of marked deficits in the water 
content of plant organs is not lacking. Livingston and Brown, 4 work- 
ing with a number of plants growing near Tuscon, Arizona, found 
that (with tin- exception of the true xerophytes as Covillea and 
Prosopis) during the afternoon the leaves suffered a marked decrease 
in water content which was made up during the night. This periodic 



2 Cairo Sci. Jour., vol. 5, p. 221, 1911. 

s Trans. Royal Soc. Can., ser. 3, vol. 10, ]>. 55, 1916, see also Bull. Torr. Bot. 
Club, vol. 40, p. 1-26, Jan., 191.1. 

•«Bot. Gaz. vol. 53, p. 319, April, 1912. 



40 University of California Publications in Agricultural Sciences [Vol. 3 

diurnal condition of dessieation has been found by Livingston and 
Brown to serve as a check on the absolute transpiration and has been 
termed "incipient drying." Lloyd 5 independently obtained similar 
results in his investigations on Fouqwieria splendens and Mrs. Shreve 6 
established the same phenomonon in 1913 with Parkinsonia micro- 
phylla. 

Inasmuch as the genus Citrus is undoubtedly a mesophyte of 
tropical origin and therefore grown in the interior valleys of Cali- 
fornia under purely artificial conditions, 7 it would naturally be 
expected that the abnormal water relations above discussed might 
obtain to an unusual degree, especially during the hot growing period, 
when the ability of the plant to make up for excessive transpiration 
is taxed to the limit. Citrus fruits are borne on wood of the current 
season's growth which ordinarily bears six to eight leaves on the same 
fruiting shoot. Therefore, it seemed reasonable that under conditions 
of excessive transpiration the leaves might draw on the water supply 
of the fruits and thus bring about an abnormal water relation. With 
the above considerations in mind it occurred to the writer that this 
premature fall of the fruits might be due to irregularities or abnormal- 
ities in the water relations between the fruits and foliage, resulting in 
abscission in some way analagous to the shedding of cotton bolls under 
the stimulus of a water deficit. 

The method used in obtaining the data here presented consisted 
in the main of simple moisture determinations of leaves and fruits of 
various kinds taken at different hours of the day. The material was 
gathered and quickly placed in weighing cups fitted with ground 
glass covers. After weighing, the material was thoroughly dried 
and then reweighed. For convenience in the case of fruits and large 
leaves, the material was cut into small pieces. The calculations are 
based on the dry weight of the material, except as otherwise stated. 
The data obtained are shown in condensed form in table 1. The 
figures shown represent averages of at least ten duplicate determina- 
tions, and in most instances of more. 

The data presented in table 1 show .some very interesting condi- 
tions. It is quite clear that, with the exception of the new succulent 
growth, the young fruits are at all times higher in water content than 
the leaves situated near them. These data also seem to leave no doubt 



5 Plant World, vol. 15, p. 11, 1912. 

o Ann. Ept. Dir. Bot. Ees. C. I. W., Feb. 12, 1913, p. 81. 

' For a more complete discussion see Livingston, B. E., "A single index to 
represent both moisture and temperature conditions as related to plants." 
Physiological Researches, vol. I, No. 9, April, 1916. 



1917] Hodgson: Abnormal Water Relations in Citrus Trees 41 

TABLE 1 

Average Moisture Content 

Average water content 

in per cent 
Kind of material (Dry weight) 

New leaves about two weeks old 242.0 

Full grown leaves of current season 's growth 162.2 

Leaves of one season's growth — about one year old 132.7 

Leaves of two season's growth — about two years old 126.1 

Leaves of three or more season 's growth. Over two years 

old ' 117.6 

Leaves of current season 's growth. Gathered between 

9 A.M. and 12 p.m 164.9 

Same gathered between 1 p.m. and 4 p.m 157.2 

Leaves of current season's growth gathered from behind 

fruits between 9 a.m. and 12 M 166.8 

Same gathered between 1 p.m. and 4 p.m 160.4 

Fruits destined to subsequent abscission, one-third to three- 
fourths inch in diameter 191.5 

Fruits apparently normal gathered between 9 a.m. and 12 m.s 260.2 

Same gathered between 1 p.m. and 4 p.m 247.7 

Fruits destined to subsequent abscission gathered between 

9 a.m. and 12 m 201.4 

Same gathered between 1 p.m. and 4 p.m 179.2 

of the fact that as the leaves grow older there is a progressive decrease 
in water content. 

It is also quite evident that a regular diurnal decrease in the water 
content of leaves of the current season's growth is manifest during 
the afternoon. Such leaves averaged 164.9% in water content for the 
period between 9 a.m. and 12 m. and only 157.2% for the period 
between 1 p.m. and 4 p.m. This difference does not appear significant 
when viewed in the light of the large differences obtained by Living- 
ston and Brown with some of their material. However, it should 
be borne in mind that those authors were dealing, for the most part, 
with much more succulent plants containing a large amount of water 
storage tissue. Further, it should be noted that these figures are 
averages, since the determinations on which they are based were not 
made at the same hours. Individual pairs of determinations fre- 
quently showed differences of as much as 25% to 30% in as short a 
period as six hours. On June 5 at 2:30 p.m., with the temperature at 
95° P and the relative humidity at 19%, the water content of leaves 
of the current season's growth was 144.3%. At 4 a.m. the next morn- 



s The fruits used for these determinations averaged a little larger than 
those gathered in the forenoon and therefore would normally be somewhat 
higher in water content. 



42 University of California Publications in Agricultural Sciences [Vol.3 

ing, with the temperature at 62° P. and the humidity 54%, the water 
content of similar leaves was found to be 172.6%, showing a differ- 
ence of 28.3%. This phenomenon is taken to indicate the presence of 
incipient drying in citrus and is in full accord with the results of 
the writers above mentioned as well as with those obtained by Lloyd. 

Since the young fruits have a higher water content than adjoin- 
ing leaves which, in turn, exhibit a diurnal decrease in relative water 
content, the conclusion, a priori, that the leaves might possibly draw 
on the water supply of the fruit during periods of excessive transpira- 
tion seemed entirely plausible. If such is the case it would seem that 
leaves so favorably situated should not show this daily variation, at 
least to the degree shown in the leaves not so favorably situated. The 
data in table 1 show, however, that the average difference in water 
content of the two sorts of leaves gathered in the forenoon and after- 
noon is quite small. This is taken to indicate that if such leaves do 
utilize the water supply of the fruits, the evaporating power of the 
atmosphere is so strong that as fast as they receive this surplus water, 
it is lost and thus causes no appreciable difference in their relative 
water content. 

The next step was to ascertain the water content of different kinds 
of fruits, those destined to remain and mature, and those showing 
indications of subsecpient abscission. It is quite easy to distinguish 
between the two, from a week to ten days before abscission occurs, 
by the difference in their appearance. Exposed fruits destined to 
drop exhibit a small yellow spot about the navel end several weeks 
before the actual drop occurs. This spot gradually extends and 
spreads until at abscission it usually occupies at least half the area 
of the fruit. In the case of well-shaded fruits, the yellow color is 
evenly distributed over the entire surface. A large number of mois- 
ture determinations were made which showed that those fruits destined 
to subsequent abscission averaged 59% less water than those fruits 
destined to remain and mature. (See table 1.) The presence of this 
condition in the fruits, especially when considered in connection with 
the daily increase at certain hours in the water deficit of the leaves 
immediately behind them, seems to point to the possibility of the leaves 
depriving the fruit of a part of their normal water supply. It cer- 
tainly indicates an abnormal water relation. 

Lemon growers prune their trees at all seasons of the year, even 
while the fruit is still on the trees. It is a well established practice to 
gather the good fruit from the excised branches immediately, in order 
to prevent it from becoming flaccid. Inasmuch as the fruit, as ordi- 



1917 J Hodgson: Abnormal Water Relations in Citrus Trees 43 

narily picked from the tree, remains turgid for several months, it is 
the common belief that the leaves draw the water out of the fruit when 
the branch is severed from the tree. That this is exactly what does 
occur, when the leaves are deprived of their normal water supply, is 
shown by the following experiments: 

Experiment 1 — Two shoots bearing small terminal oranges of 
approximately the same size and having the same number of leaves 
and approximately the same leaf area, were taken to the laboratory, 
placed on the table and allowed to dry under similar conditions except 
that in one case the fruit was severed from the stem. All cut surfaces 
were sealed with vaseline 

Within twelve hours a marked difference in appearance was 
observed. The leaves on the shoot from which the orange was detached 
were considerably shriveled while those on the other shoot remained 
turgid and fresh. This difference became more pronounced as time 
elapsed and in thirty hours a distinct difference in the appearance of 
the fruits as well as leaves was visible. The detached fruit remained 
firm and retained its dark green color and lustre while the attache, I 
fruit was soft and flaccid and exhibited a dull green color without 
lustre. This experiment was performed repeatedly with both oranges 
and lemons with the same results. (See plate 12, fig. 1.) 

As all the cut surfaces were sealed, it seems clear that the leaves 
on the shoot with fruit attached actually drew on its water content 
and that it was this supply of water which enabled them to remain 
alive and fresh long after the leaves on the other shoot had withered 
and died. 

Experiment 2 — Quantitative data on water content were desired 
to substantiate the visible indications described in Experiment 1. 
Therefore the latter was repeated several times and moisture determin- 
ations on leaves and fruits were made at various periods. A repre- 
sentative set of such determinations is given in table 2: 

TABLE 2 

Moisture Content Determinations. Twenty-four Hours After Beginning 

of Experiment 2 

Weight of Weight of Weight of Water con- 
container and same when material in tent per 
Kind of materia] fresh material dry grams cent 
in grams 

Orange detached from branch 23.40 21.670 2.665 185.0 

Orange attached to branch 23.585 22.367 2.075 142.1 

Leaves from branch C21.831 21.S05 .181 

with fruit removed [22.045 22.010 .170 21.4avg. 

Leaves from branch \ 21.345 21.275 .175 

with fruit attached ) 20.604 20.477 .284 73.7 avg. 



44 



University of California Publications in Agricultural Sciences [.Vol. 3 



These data show that after twenty-four hours the leaves on the 
shoot with orange attached contained an average of 52.3% more water 
than those on the other shoot. They further show that the detached 
fruit contained 42.9% more water than the attached fruit from which 
the leaves had been drawing their supply. This is considered to be 
conclusive evidence that in the case of excised branches the leaves 
can draw water from the fruit. 

Experiment 3 — Two shoots in every respect similar to those used 
in the previous experiments were treated in the same manner as those 
of Experiment 1 and 2. These were then weighed at irregular inter- 
vals until they had reached a constant weight. During the interim 
they were kept on the laboratory table. The data obtained are found 
summarized in table 3 : 











TABLE 3 










Water Content Determinations Made at 


Irregular 


Intervals Based on 








THE 


Whole Weight 










Shoot 


with orange 


attached 




Shoot 


with orange detached 

A 


r - 
Number 
of hours 
elapsed 



Weight 
in grams 

4.872 


Loss in 

grams 


Loss in 
per cent 


Difference 
in 
per cent 


Weight 
in grams 

4.777 


Loss in 
n grams 


Loss in 
per cent 


Difference 

in 
per cent 


3 


4.436 


.436 


8.9 


.6 


4.380 


.397 


8.3 




19 


3.957 


.915 


18.7 




3.830 


.947 


19.8 


1.1 


21 


3.895 


.977 


20.0 




3.742 


1.035 


21.7 


1.7 


24 


3.803 


1.069 


21.9 




3.607 


1.170 


24.4 


2.5 


26 


3.683 


1.189 


24.4 




3.442 


1.335 


27.9 


3.5 


27 


3.610 


1.262 


25.9 




3.342 


1.435 


30.0 


4.1 


44 


3.263 


1.609 


33.0 




2.911 


1.866 


39.1 


6.0 


49 


3.047 


1.825 


37.4 




2.682 


2.095 


34.8 


6.4 


51 


2.920 


1.952 


40.0 




2.575 


2.202 


36.0 


6.0 


91 


2.125 


2.747 


56.3 




2.008 


2.769 


57.9 


1.6 


96 


2.053 


2.819 


57.8 




1.960 


2.817 


58.9 


1.1 


99 


2.000 


2.872 


58.9 




1.935 


2.842 


59.5 


.6 


116 


1.921 


2.951 


60.5 




1.873 


2.904 


60.8 


.3 


119 


1.894 


2.978 


61.1 




1.852 


2.925 


61.2 


.1 


121 


1.881 


2.991 


61.3 




1.844 


2.933 


61.4 


.1 


140 


1.825 


3.041 


62.5 


.4 


1.807 


2.970 


62.1 




146 


1.797 


3.075 


63.1 


.5 


1.783 


2.994 


62.6 




162 


1.774 


3.098 


63.5 


.6 


1.770 


3.007 


62.9 




186 


1.736 


3.136 


64.3 


.8 


1.743 


3.034 


63.5 




195 


1.717 


3.155 


64.7 


.9 


1.726 


3.051 


63.8 




211 


1.705 


3.167 


65.0 


1.1 


1.720 


3.057 


63.9 




218 


1.695 


3.177 


65.2 


1.0 


1.710 


3.067 


64.2 




260 


1.666 


3.206 


65.8 


1.1 


1.6S0 


3.091 


64.7 




285 


1.652 


3.220 


66.0 


1.1 


1.675 


3.102 


64.9 




306 


1.642 


3.230 


66.2 


1.1 


1.664 


3.113 


65.1 




330 


1.631 


3.241 


66.5 


1.1 


1.652 


3.125 


65.4 




525 


1.613 


3.259 


66.8 


1.0 


1.631 


3.146 


65.8 





1917] 



Hodgson: Abnormal Water Relations in Citrus Trees 



45 



The data in this table indicate that the amount of water in the 
fruit available for use by the leaves was sufficient to maintain the 
latter alive for approximately 50 hours after the shoot was cut from 
the tree. It is further evident that when three hours had passed the 
leaves on the sboot with fruit attached had not yet begun to take water 
from the fruit to any appreciable extent because the shoot with fruit 
detached shows less water loss than the shoot with fruit attached. 
However, this condition was soon reversed and the leaves began to 




20 40 60 80 100 120 140 160 180 200 220 

Fig. 1. Showing the difference in per cent of water loss of shoot with 
orange attached and shoot with orange detached. The water loss curve of the 
shoot with fruit detached is considered as normal. .Ordinates represent differ- 
ences in per cent of water loss, abscissae, the time elapsed in hours. Water 
content calculated on basis of fresh weight. 



draw on the water in the fruit while the leaves to which no water was 
available from the fruit showed indications of wilting. 

That shortly after 50 hours had passed death occurred in the 
leaves of the shoot with fruit attached is shown by the rapid increase 
in the amount of water loss. This was undoubtedly due to increased 
permeability of the cytoplasmic cell membranes after death. After 
50 hours the difference in water content of the two was 18.3% in 
favor of the shoot with fruit attached. However, from this time on 
until both had reached a constant rate of water loss (after about 200 



46 University of California Publications in Agricultural Sciences [Vol. 3 

hours), this shoot lost water more rapidly than the shoot with fruit 
detached. These relations are very clearly shown in figure 1. The 
normal water loss curve is illustrated in figure 2. 

Experiment 4 — A forked twig bearing a small terminal fruit on 
each branch was selected and cut. The fruits were immediately im- 
mersed in water and the shoot tied to a support in such a fashion 
that all the leaves were exposed to the air, the fruits alone being 
immersed. One orange was now removed by cutting it under water 
and all cut surfaces were sealed. The two fruits remained under 




Fig. 2. Showing the general type of water loss curve of a shoot detached 
from the tree, including detached orange. Ordinates represent water loss in 
per cent and abscissae, the time elapsed in hours. 

water. The container and support were then placed on a bench in 
the shade in the open air and left for fifteen hours, at the end of 
which time moisture determinations were made on the fruits. 

TABLE 4 

Moisture Determinations After Fifteen Hours 







Weight of 


Weight of 


Weight of 


Water con 






container and 


same when 


material in 


tent per 


Kind of 


material 


fresh materia] 
in grams 


dry in 
grams 


grams 


cent 


Detach iM 


orange 


24.680 


21.435 


3.330 


206.9 


Attached 


orange 


23.210 


21.773 


2.570 


126.8 



1917] Hodgson: Abnormal Water Relations in Citrus Trees 47 

The data in table 4 show that at the end of fifteen hours there was 
a difference in water content between the two fruits of 80.1%. There 
seems to be no way of accounting for this large difference other than 
that the leaves had actually drawn the major part of it at least, from 
the attached fruit. 

Water Transport Studies by Means op Dye Stuff Solutions 
Experiment 3 — Bearing the foregoing findings in mind, it seemed 
desirable to determine something of the nature of this reversal of 
normal water flow by means of dye solutions. Accordingly a shoot 
bearing a terminal fruit was cut from the tree and the orange pared 
away at the apical end to open the tracheal elements and admit the 
dye. 1 ' This paring was done under the solution to prevent the entrance 
of air bubbles. Water soluble eosin was used. The orange was 
immersed in the liquid for a half hour, after winch the shoot was 
split open. The tracheal tubes throughout all parts of the leaves, 
stems and fruits were found to be strongly stained. 

E. rp, i-inii id 6 — It seemed desirable to simulate the actual situation 
on the tree as nearly as possible and the following experiment was 
designed to accomplish this. A crooked fruiting branch bearing a 
number of small lateral shoots and leaves, and one terminal orange 
was cut under water. The cut end was kept under water and the 
branch so supported that the fruit was immersed in an eosin solution. 
The apex of the orange was then pared as described above. The 
branch then rested with its basal end in water and the vascular 
bundles of the fruit open to eosin at the other end of the branch. 
(See pi. 12, fig. 2.) If we substitute for the water container the con- 
ducting system of the tree, and for the watery solution of eosin the 
developing fruit high in water content, we have very similar conditions 
to those existing in the experiment, save for the fact that the fruit is 
not open to the air and the conducting system bears a certain relation 
to the rest of the tree. 

The experiment was begun late in the afternoon and the branch 
left outdoors over night. At 8 o'clock the next morning the leaves 
were examined and found to be very fresh and turgid. Indeed they 
were noticeably much fresher in appearance than they had been the 
evening before. On careful examination absolutely no trace of eosin 



9 It should be stated here that the Washington Navel orange is in reality 
a double fruit, with a small secondary orange within a large primary fruit. 
This interior fruit constitutes what is known as the navel and it possesses an 
independent vascular system of its own which traverses the central pith of 
the primary fruit before ramifying through the secondary orange. This central 
pith thus acts as the stem to the small fruit. 



50 University of California Publications in Agricultural Sciences [Vol.3 

ing between 50-100 per square millimeter as compared to 300-450 per 
square millimeter on the leaves. Measurements of the leaves situated 
within six inches of the fruit showed that, in addition the leaf area 
immediately behind the young growing fruit is larger than the area of 
the fruit until it reaches approximately two inches in diameter, after 
which falling of the fruit is comparatively rare. Therefore, it seems 
highly probable that the transpiration of the fruit as compared to 
that of the leaves situated immediately behind it is an almost negligible 
factor and it appears reasonably certain that either water is actually 
drawn back or that the normal supply is decreased. 

Considering these two possibilities, the first merits more considera- 
tion as it is supported by proof which, though not absolute, is at least 
presumptive evidence of a strong enough character : while on the other 
hand the second possibility, agreeing though it does with the most 
recent theory on sap movements in plants as put forth by Dixon, is 
still a theoretical consideration. According to this theory, which pos- 
tulates strong tensions existing in the ascending water columns, no 
assumption of an actual reversal of the current is necessary in order 
to explain a decrease in moisture content. During normal conditions 
the relation between the tensions existing in the water columns leading 
to the fruits and those leading to the leaves is such that both organs 
receive an adequate water supply. The tension existing in any one 
of these water columns is a function of the transpiring force existing 
in the transpiring plant organ as modified by atmospheric conditions. 
Therefore, as these transpiring forces vary, the tensions vary. Trans- 
piration from the leaves, for reasons pointed out above, is subject to 
much greater variation than that from the fruits. Therefore during 
periods when evaporation is greatly accelerated the tensions in those 
water columns leading to the leaves are greatly increased and as a 
consequence more water is drawn to them. As the source of supply 
in the conducting system is practically constant, the amount in the 
fruits is thereby reduced and this results in a decrease of relative 
water content of a magnitude conditioned by the transpiration of the 
fruit, 

However, it should be noted that the data in table 1 show a 
decrease in absolute water content of the fruit of 15% to 209?, a loss 
of considerable magnitude. There are only two ways in which such 
a decrease in absolute water content can take place: (1) the water 
is lost by transpiration from the fruit, or (2) it is drawn back by 
the leaves. But since the fruit possesses a very small stomatal area 



1917] Hodgson: Abnormal Water Relations in Citrus Trees 51 

as compared with the leaves and, moreover, it is highly probable that 
a large percentage of this area is non-functional, being obstructed 
by accumulations of a resinous nature, there is small likelihood for 
absolute loss of water in this manner to the extent noted. Hence 
there seems hut one way to explain it and that is by movement back 
from the fruits. 

Evidence of an indirect nature pointing to the same conclusion 
lies in the fact that there are some indications that abscission of a 
certain proportion of the young fruits is directly due to the influence 
of hydrolysing enzymes secreted by certain saprophytic or facultative 
parasitic fungi always found present on the shriveled style and fre- 
quently in the proliferations of the navel. Such enzymes in order to 
act on the abscission layer must be drawn back through the vascular 
systems of the fruit into the pedicel where this layer is located. 
Investigations on this point are now in progress. 

Experiment 7 — Three similar fruits were selected on different 
parts of a tree; on one of the lower branches in the shade, at a height 
of four feet, and in the top of the tree in full sunlight. At noon each 
fruit was pared so as to admit entrance of a solution and then plunged 
quickly into a small vial containing a watery solution of eosin. These 
vials were securely tied to the shoot and left suspended for two hours. 
At the end of that time, on cutting leaves from these shoots, eosin 
staining was found in the vascular systems of all. On examining back- 
ward toward the tree, eosin was found as far back as thirty centi- 
meters. This experiment was repeated a number of times both at 
Edison and at Riverside and uniformly gave the same results, although 
much less marked at the latter place. In every case the backward 
movement of the eosin solution was at its maximum during the 
afternoon. 

Cutting the ends of branches in situ under a watery solution of 
eosin was tried at different times of day and gave similar results. 
This experiment was performed at Edison, Riverside and Indio. At 
the latter place, with the temperature at 116° F and the humidity at 
>'. the eosin solution traveled backward at the astonishing rate of 
30 cm. per minute at 6 p.m. Similar results were obtained using 
Eucalyptus rudis as material. In fact with long slender poles of 
Eucalyptus tereticornis at Edison, such a remarkably rapid backward 
flow of eosin was observed (105 cm. in one minute) in the afternoon 
as to compel the conclusion that after all, the force responsible for 
this movement under such conditions must be negative pressure pro- 



PLATE 12 

Fig. 1. Showing extent to which the leaves can draw on the water in the fruit. 
Both shoots were cut at the same time and had approximately the same leaf 
area. All cuts were sealed with vaseline. The fresh-appearing leaves on the 
shoot at the left have maintained themselves at the expense of water in the 
fruit. Note the difference in reflection of light from the two fruits. See 
Experiment 1. 

Pig. 2. Photograph illustrating an orange shoot so arranged as to be able to 
draw water from one end and eosin solution through the pared apex of a small 
fruit at the other. In spite of this double supply a large water deficit occurred, 
and eosin was drawn back from the container on the right to the leaf next to the 
water container on the left. See Experiment 6. 



I 54 | 



UNIV. CALIF. PUBL. AGR. SCI. VOL. 3 



[HODGSON] PLATE 12 




Fig. 1 




Fie. 2 



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